The short answer is "reciprocity failure". At low exposure rates the
effect of a single radiation event on a single halide crystal in the film
emulsion will often decay before subsequent stabilizing events hit. Ths is
reflected in the film reciprocity curve, which shows a non-linear
relationship between exposure and optical density at very low temperatures.
The decay of single event effects is temperature dependent. Thus, by
lowering the temperature, you prevent the decay and increase the sensitivity
of the film. You can get the same effect by pre-flashing the film. The
techniques was used by astronomers long before molecular biologists twigged
to it; they pack their cameras in dry ice for long exposures.
Note that this is only a significant factor if the signal is low and the
exposure has to run for days.
In Article <56vh41$292s at piglet.cc.uic.edu>, blackman at tigger.cc.uic.edu
(Samuel C. Blackman) wrote:
>My labmates and I, obviously with too much time on our hands, were
>wondering why we put our 32P-labelled autorads in the freezer at -80C.
>Our advisor hypothesized that the low temp. promotes a more "focused"
>autorad, but without a good explanation for that. The decay equation
>has no mention of temperature, so we're stumped. Any ideas?
>>-- Sam
>>>--
>Samuel C. Blackman ! InterNet : blackman at tigger.uic.edu>MD/PhD Student (4/8) ! Disclaimer: I speak for me, not UIC!
>Univ. of Ill. at Chicago ! Quote : "Quandro potro io finir di stupire?"
>Dept. of Pharmacology ! Phone : 312/996-4983 (lab) Fax: 312/996-1225
Warren Gallin
Department of Biological Sciences
University of Alberta
Edmonton, Alberta T6G 2E9
Canada
wgallin at gpu.srv.ualberta.ca